The present invention relates generally to interfaces or accessories for facilitating communications using a modem and, more specifically, to a device that allows an alternating voice-data modem (AVD) to be coupled to a digital or PBX-type telephone system via the handset jack of a telephone instrument.
A modem is a telecommunications device that is used for interfacing a computer, facsimile machine or similar data device with a telephone line to enable the data device to communicate with another like device over the telephone line. A modem typically has a data connection to the computer or other data device and a two-wire RJ-11 telephone connector for connecting the modem to the telephone line.
Practitioners in the art have developed modems known as alternating voice-data (AVD) modems that switch between a voice mode and a data mode. An AVD modem has a "Phone" jack for coupling it to a telephone instrument, as well as a "Line" jack for coupling it to the telephone line and a data connection for coupling it to the computer. The "Phone" and "Line" jacks are typically RJ-11 jacks. With the AVD modem in voice mode, a person can use the telephone instrument to conduct a conversation with another party. The AVD modem includes a number of relays, which are described in further detail below in the context of the present invention. In voice mode, the AVD modem relays loop the "TIP" and "RING" signals of the telephone instrument through the AVD modem and connect them directly to the telephone line "TIP" and "RING" signals. In other words, the AVD modem connects the "Phone" jack to the "Line" jack, thereby bypassing the internal data modulation/demodulation circuitry of the AVD modem. If the person desires to send data from his computer to another party's computer, the person terminates the conversation and, using suitable data communication software, commands his computer to send data to the AVD modem. The sending AVD modem responds to the command received from the computer by automatically switching from voice mode to data mode and then generating a special "wake-up" tone that signals the other party's AVD modem to switch to data mode. In data mode, the AVD modem relays connect the data modulation/demodulation circuitry to the telephone line and disconnect the telephone instrument from the telephone line. Although the telephone instrument cannot be used during data communication because it is disconnected from the telephone line, the AVD modem nonetheless supplies the telephone instrument with the proper voltage to enable it to remain in an operational mode. When the data transmission is complete, the AVD modem automatically switches back to voice mode. An AVD modem thus allows a person to intersperse data communication with voice communication during a telephone call.
Another scheme that allows a person to communicate both data and voice during a telephone call is known as digital simultaneous voice-data (DSVD), also known as "voice-over-data." Voice-over-data modems allow voice and data to be transmitted simultaneously in a multiplexed manner rather than in the switched manner of an AVD modem. Voice-over-data modems are used by connecting a headset, a handset or a telephone instrument to them. Thus, although a voice-over-data modem, like an AVD modem, includes a connection for coupling it to a computer, a connection for coupling it to a telephone line, and another connection for providing voice communication, they handle the voice signal in completely different ways.
A modem cannot easily be connected to a digital or PBX-type telephone system. The problem is described U.S. Pat. No. 4,907,267, issued to Gutzmer, entitled "MODEM INTERFACE DEVICE." In a PBX telephone system, multiple lines are serviced by a central controller. It is difficult to achieve dedicated line service for a modem because the telephone line to which a particular telephone instrument is connected carries digital line selection information and power in addition to the audio frequency communication signal of the handset. To solve this problem, the above-referenced U.S. patent describes a solution in which a modem is connected to the telephone system via the handset jack of the telephone instrument base unit. The patent describes an interface device that allows a user to switch between voice and data communication. To use the interface device, both the modem and the handset that was unplugged from the telephone instrument are plugged into the device. The device is then plugged into the handset jack of the telephone instrument base. The device has a switch that allows the user to selectively connect either the handset or the modem to the handset jack. The interface device converts the two-wire modem connection to the four-wire handset connection using a transformer. The transformer also provides impedance matching.
An interface device for connecting a voice-over-data modem to a digital or PBX-type telephone system is described in U.S. Pat. No. 5,455,859, also issued to Gutzmer, entitled "TELEPHONE HANDSET INTERFACE FOR DEVICE HAVING AUDIO INPUT." The problem solved by that interface device is to provide proper DC bias voltages to the headset or handset microphone while routing the audio frequency voice signals from the microphone and speaker to the telephone line. An AVD modem does not exhibit this problem because it is connected to a complete telephone instrument rather than to a headset or handset by itself. Nevertheless, there are other problems that must be solved in order to provide an interface device for connecting an AVD modem to a digital or PBX-type telephone system.
An AVD modem typically automatically switches between data mode and voice mode in the manner described above only if the telephone instrument remains off-hook, although certain AVD modems allow themselves to be set up via software to override this requirement. This requirement is not a problem when an AVD modem is used in the conventional manner with a standard analog (non-PBX) telephone. The user simply does not return the handset to the cradle when initiating a data transmission. The requirement that the telephone instrument remain off-hook presents problems, however, to interfacing the AVD modem to a digital or PBX-type telephone system.
A modem interface device for connecting an AVD modem to a digital or PBX-type telephone system is known as INSIDE LINE.TM. and is produced by Radish Communications Systems, Inc. The INSIDE LINE.TM. interface device utilizes the software set-up feature of certain AVD modems noted above. The INSIDE LINE.TM. interface device thus operates only in conjunction with special software executing on the computer. The software generates a command, in response to which the AVD modem disables its circuitry for sensing whether the telephone instrument is on-hook or off-hook.
It would be desirable to provide an interface device for connecting an AVD modem to a digital or PBX-type telephone system that does not require special software executing on the computer. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.